1. Field of the Invention
The present invention relates to a magnetoresistive element and a magnetoresistive random access memory including the magnetoresistive element.
2. Related Art
In recent years, a number of solid-state memories that record information have been suggested on the basis of novel principles. Among those solid-state memories, magnetoresistive random access memories (hereinafter also referred to as MRAMs) that take advantage of tunneling magneto resistance (hereinafter also referred to as TMR) have been known as solid-state magnetic memories. Each MRAM includes magnetoresistive elements (hereinafter also referred to as MR elements) that exhibit magnetoresistive effects as the memory elements of memory cells, and the memory cells store information in accordance with the magnetization states of the MR elements.
Each MR element includes a magnetization free layer having a magnetization where a magnetization direction is variable, and a magnetization reference layer having a magnetization of which a direction is invariable. When the magnetization direction of the magnetization free layer is parallel to the magnetization direction of the magnetization reference layer, the MR element is put into a low resistance state. When the magnetization direction of the magnetization free layer is antiparallel to the magnetization direction of the magnetization reference layer, the MR element is put into a high resistance state. The difference in resistance is used in storing information.
As a method of writing information on such a MR element, a so-called current-field write method has been known. By this method, a line is placed in the vicinity of the MR element, and the magnetization of the magnetization free layer of the MR element is reversed by the magnetic field generated by the current flowing through the line. When the size of the MR element is reduced to form a small-sized MRAM, the coercive force Hc of the magnetization free layer of the MR element becomes larger. Therefore, in a MRAM of the current-field write type, the current required for writing tends to be larger, since the MRAM is small-sized. As a result, it is difficult to use a low current and small-sized memory cells designed to have capacity larger than 256 Mbits.
As a write method designed to overcome the above problem, a write method that utilizes spin momentum transfers (SMT) (a spin-injection writing method or spin-transfer-torque writing method) has been suggested (see U.S. Pat. No. 6,256,223). By the spin-injection write method, a current is applied in a direction perpendicular to the film plane of each of the films forming a MR element having a tunneling magnetoresistive effect, so as to change (reverse) the magnetization state of the MR element.
In a magnetization reversal caused by spin injection, the current Ic required for the magnetization reversal is determined by the current density Jc. Accordingly, as the area of the face on which the current flows becomes smaller in a MR element, the injection current Ic required for reversing the magnetization becomes smaller. In a case where writing is performed with fixed current density, the current Ic becomes smaller, as the size of the MR element becomes smaller. Accordingly, the spin-injection writing method provides excellent scalability in principle, compared with the current-induced magnetic field writing method.
In a MRAM of a spin injection type, however, the current that can be applied at the time of writing is determined by the voltage generated at a selective transistor and the relationship in resistance between the selective transistor and each TMR element. Therefore, it is necessary to lower the resistance of each TMR element, or to lower the resistance of each TMR film.